Lanthanoids – Electronic Configuration, Oxidation States and Lanthanoid Contraction

What are Lanthanides Elements?

Lanthanides (Rare earth elements )  made up the modern periodic table, which includes elements with atomic numbers ranging from 58 to 71 after Lanthanum. Rare earth metals are so named because they only make up a small percentage of the Earth’s crust (3% percent). As lanthanide orthophosphates, they can be found in monazite sand. In the year 1925, the Norwegian mineralogist Victor Goldschmidt coined the name “lanthanide.” All but one of the fifteen metallic elements in the lanthanide family (from lanthanum to lutetium) are f-block elements. These elements’ valence electrons are in the 4f orbital. Lanthanum, on the other hand, is a d-block element with a [Xe]5d16s2 electronic configuration.

Lanthanides are extremely dense elements, ranging in density from 6.1 to 9.8 grammes per cubic centimetre. These elements, like other metals, have extremely high melting and boiling temperatures (varying from 800 to 1600 degrees Celsius) (ranging from roughly 1200 to 3500 degrees Celsius). Ln3+ cations are known to form in all lanthanides.

Lanthanide Applications

Lanthanides are extremely dense metals with melting points that are even higher than those of the d-block elements. They are mixed with other metals to make alloys. These are the inner transition metals, which are also known as the f block elements. In the inner transition elements/ions, electrons can be found in the s, d, and f orbitals.

Lanthanide Series Properties

If we add the lanthanides and actinides series in the periodic table for transition metals, the table will be too lengthy. These two series, known as the 4f series (Lanthanods series) and 5f series, are found at the bottom of the periodic table (Actanoids series). Inner transition elements are the 4f and 5f series put together.

In terms of chemical and physical properties, all of the elements in the series are quite similar to lanthanum. The following are some of the most important qualities and properties:

1. They have a sheen to them and appear silvery.
2. They are soft metals that can be easy via a knife.
3. Depending on their basicity, the elements have varied response tendencies. 
4. If polluted with other metals or non-metals, lanthanides can corrode or become brittle.
5. They almost all combine to generate a trivalent compound. They can also produce divalent or tetravalent compounds  sometimes.
6. They are magnetic

Contraction of Lanthanide

Due to increased nuclear charge and electrons entering the inner (n-2) f orbitals, the atomic size of ionic radii of tri positive lanthanide ions drops continuously from La to Lu. Lanthanide contraction is the progressive decrease in size as the atomic number increases.

Lanthanide Contraction’s Consequences

The effect of lanthanide contraction will be depicted in the following points:

• Size of an atom(Atomic size
• Difficulty in lanthanide Separation
• Effect n oHydroxides’ basic strength .
• Complicated Formation
• The ionisation energy of elements in the d-block

1. Atomic size: The size of the third transition series atom is roughly identical to that of the second transition series atom. For example, the radius of Zr equals the radius of Hf, and the radius of Nb equals the radius of Ta, and so on.
2. Difficulty in separating lanthanides: Because the ionic radii of lanthanides differ only slightly, their chemical characteristics are comparable. This makes it difficult to separate elements in their pure condition.
3. Effect on hydroxide basic strength: As the size of lanthanides drops from La to Lu, the hydroxides’ covalent character increases, and their basic strength diminishes. As a result, La (OH)3 is the most basic, whereas Lu(OH)3 is the least basic.
4. Complex formation: The tendency to develop coordinates is because of the smaller size but larger nuclear charge. From La3+ to Lu3+, the number of complexes increases.
5. Electronegativity: From La to Lu, it increases.
6. Ionization energy: Because the nuclear charge attracts electrons much more strongly, the Ionization energy of 5d elements is much higher than that of 4d and 3d elements. Other than  Pt and Au, all elements in the 5d series have a filled s-shell.Ionization Energy is the same for all elements from Hafnium to Rhenium, and it increases with the number of shared d-electrons beyond that, with Iridium and Gold having the highest Ionization Energy.
7. Complex Formation: Lanthanides with a 3+ oxidation state have a bigger charge to radius ratio and hence have a lower charge to radius ratio. When compared to d-block elements, this reduces the capacity of lanthanides to form complexes. They still form compounds with powerful chelating agents such as EDTA, -diketones, and oxime, among others. P-complexes are not formed by them.

Lanthanide Electronic Configuration

Promethium (Pm) with atomic number 61 is the only synthetic radioactive element of the fourteen lanthanides having a terminal electronic configuration of [Xe] 4f1-14 5d 0-16s2. Because the energies of the 4f and 5d electrons are nearly identical, the 5d orbital remains unoccupied and the electrons enter the 4f orbital.

The exceptions are gadolinium (Z = 64), where the electron enters the 5d orbital due to the existence of a half-filled d-orbital, and lutetium (Z = 71), where the electron enters the 5d orbital because of the presence of a half-filled d-orbital.

Lanthanide Oxidation State

The oxidation state of all elements in the lanthanide series is +3. Some metals (samarium, europium, and ytterbium) were previously thought to have +2 oxidation states. Further research on these metals and their derivatives has revealed that in solution, all metals in the lanthanide class have a +2 oxidation state.

A few metals in the lanthanide class have +4 oxidation states on rare occasions. The great stability of empty, half-filled, or filled f-subshells is responsible for the uneven distribution of oxidation states among metals.

The oxidation state of lanthanides is affected by the stability of the f-subshell in such a way that the +4 oxidation state of cerium is preferred because it acquires a noble gas configuration, but it reverts to a +3 oxidation state and thus acts as a strong oxidant that can even oxidise water, though the reaction is slow

Europium (atomic number 63) has the electronic structure [Xe] 4f7 6s2. It loses two electrons from the 6s energy level and achieves the extremely stable, half-filled 4f7 configuration, allowing it to form Eu2+ion easily. Eu2+ then oxidises to the common lanthanide oxidation state (+3) and creates Eu3+, which acts as a powerful reducing agent.

In the Yb2+ form, Ytterbium (atomic number 70) possesses a filled f-orbital, making it a potent reducing agent as well. The presence of an f-subshell has a significant impact on the oxidation state and characteristics of these metals. Discoveries and advancements continue to add to the body of knowledge about lanthanides.

Unlike the d-block elements, the energy gap between 4f and 5d orbitals is considerable, limiting the number of oxidation states.

Why does the oxidation state of lanthanide vary?

Lanthanides have a wide range of oxidation states. They also show oxidation states of +2, +3, and +4. Lanthanides, on the other hand, have the most stable oxidation state of +3. As a result, elements in other states try to lose or gain electrons to reach the +3 state. As a result, those ions become powerful reducing or oxidising agents.

Aqueous Solution Oxidation State

Sm2+, Eu2+, and Yb2+ lose electrons in an aqueous solution and get oxidised, making them good reducing agents. Ce4+, Pr4+, and Tb4+, on the other hand, gain an electron and are good oxidizers. Only oxides allow for higher oxidation states (+4) of elements. Pr, Nd, Tb, and Dy are a few examples.

Lanthanide Chemical Reactivity

The reactivity of all lanthanides is similar, however, it is higher than that of the transition elements. Except for CeO2, which interacts with hydrogen at 300-400 C to generate solid hydrides, they easily tarnish with oxygen

Water causes hydrides to break down. Halides are created by heating metals or oxides with halogen or ammonium halide. Fluorides are insoluble, but chlorides are liquescent. In water, nitrates, acetates, and sulphates are soluble, but carbonate, phosphate, chromates, and oxalates are not.

Lanthanide Ionization Energy

Ionization energy is the amount of energy required to remove the valence electron from an atom/ion, and it is proportional to the electron’s force of attraction. As a result, the ionisation energy increases as the nuclear charge and electron radii decrease (IE). In addition, the ionisation energy for half-filled and filled orbitals will be higher.

Lanthanide Physical Properties

1. Density: Because density is the ratio of a substance’s mass to its volume, d-block elements will have a higher density than s-block components. The density trend in the inner transition series will be the inverse of atomic radii, i.e. density will increase as the atomic number grows over time. Their density is high, ranging from 6.77 to 9.74 g cm-3. It rises as the number of atoms in the nucleus rises.
2.  Melting and Boiling Points: Lanthanides have a very high melting point, although their melting and boiling points show no discernible trend.
3. Magnetic Properties: Materials are classed as follows based on how they interact with a magnetic field:

• • If repelled, diamagnetic
  • If attracted, it becomes paramagnetic.

Because of unpaired electrons in orbitals, lanthanide atoms/ions other than f0 and f14 are paramagnetic. As a result, the diamagnetic elements Lu3+, Yb2+, and Ce4+ exist.

The “orbital magnetic moment” and the “spin magnetic moment” are both affected by unpaired electrons. The total magnetic moment is calculated using the orbital angular moment and spin magnetic moment of the electrons.

[4S(S+1)+L(L+1)] M = [4S(S+1)+L(L+1)] BM stands for Bohr Magneton, and its unit is the Bohr Magneton (BM)

The Production of Colored Ions

Like the d-block elements, lanthanides ions can have electrons in f-orbitals as well as empty orbitals. When a frequency of light is absorbed, the light transmitted has a complementary colour to the absorbed frequency. Inner transition element ions can absorb visible frequency and utilise it for f-f electron transitions, resulting in visible colour.

Lanthanide Applications

• Metallurgical applications: Some lanthanide element alloys are used as reducing agents in metallurgical processes. For example (Ce- 30 to 35 per cent)
• Ce(III) and Ce(IV) oxides are employed in glass polishing powders, whereas Nd and Pr oxides are widely used in glass colouring and the manufacture of standard light filters.
• Catalytic uses: Lanthanide compounds are employed as catalysts in a variety of applications. As an example, cerium phosphate is utilised as a catalyst in petroleum cracking.

CONCLUSION:-

The 14 elements with atomic numbers 58 through 71 that follow lanthanum on the periodic table are known as the lanthanide series. Due to similarity in features that define each group, these 14, together with the actinides (atomic numbers 90 through 103), are excluded from the periodic table.